In this specification unless the contrary is expressly stated, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not to be construed as an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned.
Most commercial metal detectors operate in, comprising most handheld metal detectors designed to hunt for buried metal targets such as gold, coins, treasure and archaeological artifacts, means that output signal responds within a relatively short delay to any change in input signal; any delay being typically less than 1/10th of a second.
Most metal detectors have a transmitter whose output is connected to a transmit coil for the transmission of alternating magnetic fields, a magnetic field receiving means such as a receive coil which is connected to signal processing and assessment electronics The signal processing and assessment electronics usually comprises a preamplifier whose output is connected to synchronous demodulators whose synchronous demodulation multiplicatior functions are synchronised to the transmitted alternating magnetic fields. The outputs of the synchronous demodulators are connected to low-pass filters cr “demodulation filters” whose outputs are further processed for target identification and indication.
Some commercially available sinusoidal single-frequency transmitting metal detectors have switches that allow a user to select different frequencies. The electronics in such detectors is often relatively expensive. The ability to select different frequencies may be useful, for example, in gold nugget prospecting where the size of gold nuggets may vary from location to location and hence the optimal frequency for detection may also correspondingly vary.
U.S. Pat. No. 5,537,041 discloses a metal detector which transmits multi-period pulses and operates in the time-domain, as too do some commercially available pulse induction metal detectors; see for example U.S. Pat. Nos. 4,868,504 and 5,576,624.
U.S. Pat. No. 4,628,265 discloses a frequency-domain metal detector which applies a voltage square-wave signal to a transmit coil.
It is an object of the present invention to provide a metal detector that overcomes or at least substantially ameliorates the problems associated with the prior art.
Other objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.
It is a further object of some aspects of this invention to provide an improved continuous transmit current (cw), low cost metal detector capable of generating at least stronger high frequency components than existing square-wave or rectangular-wave metal detectors.
A yet further object of some aspects of this invention is to provide an improved flexible metal detector platform at relatively low cost, that assists an operator such as a prospector in the location of metal targets, in particular higher sensitivity to faster decay time constant targets.